An ideal black body at thermal equilibrium emits radiation in all frequency ranges. It emits more energy as the frequency increases. By calculating the total amount of radiated energy, it can be shown that a blackbody would release an infinite amount of energy. This contradicted the principles of conservation of energy and showed a new model was needed for the behaviour of blackbodies. This happened around the end of the nineteenth century. It was one of the things that led to the invention of quantum mechanics.
It led theorists like Max Planck to come up with quantum theory in which energy leaves the body in distinct packets called quanta rather than in continuous waves. The quantum theory accurately predicted the energy of the radiation that is observed.
Rayleigh-Jeans formula predicts that the radiated intensity should increase as square of frequency. So energy density (intensity) would become infinitely large at the high frequency end of the spectrum, but experimentally observed that it approaches zero. This divergence for higher frequencies is called ultraviolet catastrophe.